1. Field of the invention
The present invention relates to an X-ray lithography technique which is used to replicate a fine pattern for the fabrication of semiconductor integrated circuit devices and more particularly to an X-ray lithography mask with a high degree of contrast and a submicron pattern and a process for manufacturing the same.
2. Description of the prior art
As is well known in the art, X-ray lithography is a technique in which soft X-rays having wavelengths from 4 to 50 .ANG. are used as an exposing ray source so that fine patterns having submicron dimensions can be replicated. In general, an X-ray mask comprises an absorber pattern which absorbs soft X-rays and a mask substrate which supports the absorber pattern and which permits the transmission of soft X-rays.
As materials for absorber patterns, it is required to use materials which sufficiently absorb soft X-rays. If the wavelength of a soft X-ray is determined, the quantities of soft X-rays absorbed by an X-ray absorber can easily be calculated based on an X-ray absorption coefficient. FIGS. 1A, 1B and 1C illustrate the relationship of X-ray attenuation with thicknesses of various films in the cases of Mo-L radiation (5.41 .ANG.), Si-K radiation (7.13 .ANG.) and A1-K radiation (8.34 .ANG.), respectively.
As is clear from FIGS. 1A-1C, it is readily understood that elements of higher atomic numbers such as gold Au, tantalum Ta, tungsten W, rhenium Re or the like must be used in order to obtain an X-ray attenuation of the order of 10dB, so that a sufficient degree of mask contrast is attained.
For instance, Laid-Open Japanese Patent Application No. 54-141571 (Japanese Patent Application No. 53-48717), entitled "Mask for Soft X-ray Lithography" discloses an X-ray mask of the type in which fine pattern apertures through which soft X-ray pass are formed in a strained thin film formed from a soft X-ray absorbing material such as gold Au, platinum Pt, palladium Pd, tungsten W, tantalum Ta, holmium Ho, erbium Er, uranium U or the like.
Laid-Open Japanese Patent Application No. 54-11677 (Japanese Patent Application No. 53-71437) corresponding to U.S. patent application Ser. No. 810,469, entitled "Mask for Use in Fine Line Lithography and Method for Fabricating the Same", discloses an X-ray mask comprising a mask substrate in the form of a thin membrane or film which is formed from a polymer such as parylen (trademark of Union Carbide Corporation) and which is transparent to a chemical radiation and chemical radiation absorber consisting of an oxide of a rare earth element, or an element having a high atomic number and a high density such as gold Au, platinum Pt, uranium U, indium In or the like. It also discloses an ion etching process, an electroplating process and a lift-off process for forming absorbing layers of a metal having a high density such as gold Au, platinum Pt, uranium U or the like. However, with these processes it is extremely difficult to produce a pattern by using a metal having a high density.
John N. Randall, et al. disclose in "High-resolution pattern definition in tungsten" (Applied Physics Letters 39(9), Nov. 1, 1981 pp. 742-743) a mask structure in which a glass substrate is coated with an aluminum film over which a tungsten film is deposited, so that the tungsten film is patterned to form a fine pattern by reactive sputter etching. However, they only disclose the experiments made on the tungsten films deposited on the glass substrates, but have not disclosed how internal stresses can be reduced when tungsten, i.e., a metal having a high melting point, is deposited on a glass substrate. Therefore, the mask has not been used in practice.
In practice, only gold Au is used as an absorber material. The reason follows. When Ta, W, Re or the like which has a high melting point is deposited in the form of a thin film, high stresses are produced so that the thin mask substrate is damaged and distorted.
Therefore, gold Au which is relatively easily processed has been used as an absorber material. When gold Au is used as an absorber material, the gold film must have a thickness of about 0.52 .mu.m for Al-K radiation (8.34 .ANG.) and about 0.68 .mu.m for the Si-K radiation (7.13 .ANG.) in order to obtain a mask contrast of 10dB. It follows, therefore, that the aspect ratio becomes higher than 1 so that the pattern width is 0.5 .mu.m.
There are two conventional processes for providing gold absorbers. Namely, in one process an insulating film is processed to form an electroplating mask so that gold is plated on the mask substrate. The other process is the ion etching method.
The former process in which fine gold patterns are electroplated through a mask of insulating material can produce submicron patterns having steep profile or steeply vertical side walls. However, when a gold absorber has complicated patterns including mixtures of different sizes, a uniform current density distribution cannot be obtained. As a result, a portion having a pattern which is small in size becomes nonuniformly thin in its thickness dimension. Furthermore, it is difficult to control the quality of the plating solution, which causes fluctuations in the quality of the gold absorber patterns. Moreover, many fabrication steps are required in this process. The latter process in which a gold absorber pattern is formed by ion etching will be explained with reference to FIGS. 2A-2H. In FIG. 2A, a mask substrate 2 made of a material such as SiN or Si.sub.3 N.sub.4 capable of transmitting X-rays is arranged on a silicon wafer 1 and then a thin tantalum or titanium primary coat layer 3 is deposited on the mask substrate 2 so that the adhesion between the mask substrate 2 and a gold X-ray absorber layer is ensured. Thereafter, as shown in FIG. 2B, gold which acts as an absorber is deposited on the undercoating layer 3 to form an X-ray absorber layer 4.
As shown in FIG. 2C, a metal layer 5 of titanium or tantalum is deposited on the surface of the X-ray absorber layer 4 on which a resist layer 6 is formed, as shown in FIG. 2D. Thereafter, a desired pattern is transferred to the resist layer 6 by exposure and then the patterned resist layer 6 is developed so that a desired resist pattern 6' is formed, as shown in FIG. 2E.
Next, an etching process using the plasma of CF.sub.4 is applied to the metal layer 5 through the resist pattern 6' as a mask, so that an etching mask 5' as shown in FIG. 2F is formed from the metal layer 5. Thereafter, the resist pattern 6' is removed and then an ion etching process using the ions of an inert gas such as argon is applied to the X-ray absorber layer 4 through the etching mask 5' so as to obtain an X-ray absorber pattern 4' having the desired pattern from the X-ray absorber layer 4, as shown in FIG. 2G. Subsequently, the undercoating layer 3 is ion-etched.
Thereafter, the wafer 1 is etched through a mask, so that a silicon frame 1' is formed in the periphery of the Si wafer 1, as shown in FIG. 2H. In this way, an X-ray mask is fabricated.
According to the above-described ion etching process, the etched gold particles are redeposited on the side walls of the pattern 4' and the metal etching mask 5' made of titanium or tantalum is greatly retarded due to the ion etching. Consequently, the angle of inclination of the side walls of the X-ray absorber pattern 4' is of the order of 75.degree. as shown in FIG. 2G.
Thus, with the prior art ion etching process it has been difficult to form a fine gold X-ray absorber pattern of submicron order which has a sufficient contrast.
In order to overcome the above and other problems encountered in the ion etching process, there has been proposed a reactive sputter etching process (See Applied Physics Letters, 39(9), pp. 742-743 and 41(1), pp. 247-248) by which tungsten absorber patterns are formed using a mixture gas of SF.sub.6 and O.sub.2.
Stresses in the tungsten layer cannot be sufficiently relieved, so that the tungsten layer is deposited on an aluminum undercoating layer deposited on a glass substrate. Even when a polyimide membrane is used as a practical mask substrate, an aluminum undercoating layer of the thickness of 100 .ANG. is deposited on the substrate and then a tungsten layer of 800 .ANG. in thickness is deposited on the aluminum layer. In this case, the internal stress can not be sufficiently relieved and sufficient contrast cannot be obtained, so that the mask cannot be used satisfactorily in practice.
Furthermore, in order to practically form an X-ray lithography mask by the tungsten layer, the aluminum undercoating layer must be removed for the purpose of alignment, but it is very difficult to remove the aluminum undercoating layer without causing any damage to the tungsten X-ray absorber pattern. As a result, the X-ray mask cannot be satisfactorily used in practice.
Tantalum Ta is very advantageous as an absorber material for an X-ray mask.
However, tantalum Ta has a high melting point, so that it is difficult to relieve internal stresses in a tantalum layer. A tantalum layer having a high internal stress tends to be separated from the mask substrate or to cause greater distortions in the mask substrate. In the worst case, the internal stress in the tantalum layer damages the mask substrate. With such a tantalum layer having a high internal stress, it has been impossible to provide an X-ray lithography mask formed by a single tantalum layer. If such a single tantalum layer could be advantageously produced the fabrication of an X-ray lithography mask would be much facilitated.